1. Field of the Invention
The present invention relates to a data communication method of an error control for automatic repeat request system and a data communication apparatus therefor.
2. Description of the Related Art
An error control method for data communication includes an error control method of an automatic repeat request (ARQ) system suitable for application to a mobile data communication system in which a burst error occurs relatively frequently. According to this error control method of an automatic repeat request system, if a data frame transmitted from a transmission side communication apparatus (data communication apparatus) to a reception side transmission apparatus (data communication apparatus) has an error, then the transmission side communication apparatus transmits the data frame to the reception side communication apparatus again based on a re-transmission request from the reception side communication apparatus to the transmission side communication apparatus.
Although there are various kinds of error control methods of automatic repeat request, an error control method of an automatic repeat request system of a selective repeat (SR) system providing a high throughput will hereinafter be described by way of example. In theory, an apparatus employing the SR system requires a buffer memory of infinite size. Therefore, usually such an error control method of an automatic repeat request system of an SR system is not employed, but an improved method thereof is employed. For simplification of description, an error control method of automatic repeat request of an original SR system will be described.
An arrangement of an ARQ frame having the same format when it is a data frame and a feedback frame will be described with reference to FIG. 1A. This ARQ frame is formed of, for example, a 16-bit transmission data length indicating area a, an 8-bit transmission frame number area b, an 8-bit repeat request frame number area c, a 592-bit transmission data area d, and a 16-bit error detection code area e for all the areas a to d. In this arrangement, one frame is formed of 640 bits.
The transmission data length indicating area a is an area for indicating a data amount of the transmission data area d. This data amount can be set within the range from 0 to 592 bits, and in this case the data amount of the transmission data area d is 592 bits. The transmission frame number area b indicates a number of a frame to be transmitted (e.g., frame numbers 0 or 1 to 255). The repeat request frame number area c is used in a feedback frame transmitted from the reception side communication apparatus to the transmission side communication apparatus and indicates a number of a frame required to be transmitted again by the reception side communication apparatus (the smallest frame number among numbers of frames which the reception side communication apparatus has not received yet).
The error detection code e is a cyclic redundancy check (CRC) code (16 bits) according to ITU-T recommendations. This CRC code affects a 624-bit area from the transmission data length indicating area a to the transmission data area d.
An ARQ transmission chart according to the SR system presented when a transmission frame has an error will be described with reference to FIG. 1B. In FIG. 1B, numerals in rectangular frames of the data transmission side (transmission side communication apparatus) depict frame numbers of transmission frames. It is assumed that there are ten transmission frames 1 to 10 and the frame numbers 1 and 10 respectively depict the first transmission frame number and the last transmission frame number. An arrow pointing toward the lower right depicts a transmission frame transmitted from the transmission side communication apparatus to the reception side communication apparatus. An arrow pointing toward the upper right depicts a feedback frame transmitted from the reception side communication apparatus to the transmission side communication apparatus. A solid-line arrow depicts a frame having no transmission error. A broken-line arrow depicts a frame having a transmission error. A repeat request frame number of the feedback frame is depicted by a reference symbol R marked with a frame number. A reference symbol d depicts a round trip delay amount which indicates a time required for the transmission side communication apparatus to, after transmitting a transmission frame (data frame) therefrom to the reception side communication apparatus, receive a feedback frame from the reception side communication apparatus indicating whether or not the reception side communication apparatus has received the transmission frame correctly. In this example shown in FIG. 1B, the round trip delay amount d corresponds to four frame periods required for the transmission side communication apparatus to transmit four transmission frames.
Operations of the transmission side communication apparatus and the reception side communication apparatus will hereinafter be described with reference to FIG. 1B.
Operation of Transmission Side Communication Apparatus
(1) Transmit transmission frames 1, 2, 3 and 4 successively. PA0 (2) Receive a feedback frame including a response to the transmission frame 1 while transmitting the transmission frame 4. Since the repeat request frame number indicated by the feedback frame is 2, determine that the reception side communication apparatus has received the transmission frame 1 correctly. PA0 (3) Receive a feedback frame including a response to the transmission frame 2 while transmitting a transmission frame 5. Since the repeat request frame number indicated by the feedback frame is 3, determine that the reception side communication apparatus has received the transmission frame 2 correctly. PA0 (4) Receive a feedback frame including a response to the transmission frame 3 while transmitting a transmission frame 6. Since the repeat request frame number indicated by the feedback frame is 4, determine that the reception side communication apparatus has received the transmission frame 3 correctly. PA0 (5) Receive a feedback frame including a response to the transmission frame 4 while transmitting a transmission frame 7. Since the repeat request frame number indicated by the feedback frame is 4 again, determine that the reception side communication apparatus has not received the transmission frame 4 correctly. PA0 (6) Transmit the transmission frame 4 again. PA0 (7) Since it is determined after four frame periods whether or not the reception side communication apparatus correctly receives the re-transmitted frame 4 transmit the succeeding frames 8, 9 and 10 successively. PA0 (8) Receive the feedback frame including a response to the re-transmitted frame 4 while transmitting the last transmission frame 10. Since the repeat request frame number indicated by the feedback frame is 4, determine that the reception side communication apparatus has not yet received the transmission frame 4 correctly. PA0 (9) Transmit the transmission frame 4 again. PA0 (10) Since it is determined after four frame periods whether or not the reception side communication apparatus correctly receives the re-transmitted frame 4 transmit the succeeding frames 5, 6 and 7 successively. PA0 (11) Receive the feedback frame including a response to the re-transmitted frame 4 while transmitting the last transmission frame 7. Since the repeat request frame number indicated by the feedback frame is 9, determine that the reception side communication apparatus has received the transmission frame 4 correctly and that the reception side communication apparatus has not received the transmission frame 9 correctly. PA0 (12) Transmit the transmission frame 9 again. PA0 (13) Since it is determined after four frame periods whether or not the reception side communication apparatus correctly receives the re-transmitted frame 9 transmit the succeeding frame 10. PA0 (14) Since there is no frame to be transmitted after transmission of the transmission frame 10, transmit again the transmission frame 9 which has been requested once. PA0 (15) Transmit the transmission frame 10. Receive a feedback frame including a response to the transmission frame 9 while transmitting the transmission frame 10. Since the repeat request frame number indicated by the feedback frame is 10, determine that the reception side communication apparatus has received the transmission frame 9 correctly. PA0 (16) Since there is no frame to be transmitted after transmission of the transmission frame 10, transmit again the transmission frame 10 which is requested at present. PA0 (17) Receive a feedback frame including a response to the transmission frame 10 while transmitting the transmission frame 10. Since the repeat request frame number indicated by the feedback frame is 11, determine that the reception side communication apparatus has received the transmission frame 10 correctly. Thus, the transmission side communication apparatus finishes its transmission of the transmission frames 1 to 10. PA0 (1) Since having received the transmission frames 1, 2 and 3 correctly, transmit successively feedback frames having the frame numbers 2, 3 and 4. PA0 (2) Since having not received the transmission frame 4 correctly, repeatedly transmit the feedback frame whose repeat request frame number is 4 until correct reception of the transmission frame 4 occurs. PA0 (3) Since having received the transmission frame 4 correctly, repeatedly transmit a feedback frame including a repeat request frame number set to a frame number next to 8 which is a maximum number of frame numbers of the correctly received transmission frames, i.e., a frame number 9 which is a minimum number among frame numbers of frames that have not been received, until receiving the transmission frame 9 correctly. PA0 (4) Since having received the transmission frame 9 correctly, transmit the feedback frame including a repeat request frame number set to a frame number 10 in dictating the transmission frame requested to be transmitted next. PA0 (5) Since having received the transmission frame 10 correctly, transmit the feedback frame including a repeat request frame number set to a frame number 11 indicting of the transmission frame requested to be transmitted next.
Operation of Reception Side Communication Apparatus
In the ARQ error control method of the SR system shown in FIG. 1B, since the transmission side communication apparatus transmits the transmission frames 1 to 10 to the reception side communication apparatus, the number N1 of the transmission frames practically transmitted is 16, while the number N2 of the transmission frames transmitted until the transmission side communication apparatus receives all the feedback frames for confirming correct transmission of all the transmission frames 1 to 10 after transmitting the first transmission fame is 20.
A throughput S defined by a ratio N0/N1 (N0 is the number of the transmission frames to be transmitted and N1 is the number of all the transmission frames practically transmitted from the transmission side communication apparatus to the reception side communication apparatus) is S=10.div.16=0.625 in the example shown in FIG. 1B.
In the example shown in FIG. 1B, since the transmission side communication apparatus twice failed to correctly transmit the transmission frame 4 to the reception side communication apparatus and correctly transmitted the transmission frame 4 thereto once, it transmitted the transmission frame 4 thereto three times in total. Since the transmission side communication apparatus once failed to correctly transmit the transmission frame 9 to the reception side communication apparatus and correctly transmitted the transmission frame 9 thereto once, it transmitted the transmission frame 9 thereto two times in total. Further, although the transmission frames 5, 6, 7 could correctly be transmitted from the transmission side communication apparatus to the reception side communication apparatus successively after transmission of the transmission frame 4, they were transmitted again because of the round trip delay. The reason for this re-transmission results from the fact that the repeat request frame numbers of the feedback frames are 4 until the transmission frame 7 is transmitted again. This means that some of the transmission frames are transmitted uselessly. Thus, in the example shown in FIG. 1B, the throughput is considerably lowered. In general, the larger the round trip delay amount becomes, the more the throughput is lowered.
A general operation of the above ARQ transmission of the SR system shown in FIG. 1B will be described. Initially, the transmission side communication apparatus transmits the frames from the first predetermined frame number to the last predetermined frame number to the reception side communication apparatus in ascending order (or descending order). The reception side communication apparatus transmits a feedback frame including a repeat request frame number FB1(n) formed of a minimum frame number FBM(n) among frames which have not been received and having the same format as that of a transmission frame, to the transmission side in response to the frame numbers of the received first to nth frames. The transmission side communication apparatus transmits a transmission frame which the reception side communication apparatus has not correctly received, to the reception side communication apparatus in response to the repeat request frame number FB1(n) of the received feedback frame.
As described above, in a data communication method employing the ARQ transmission shown in FIGS. 1A and 1B, even when the transmission side communication apparatus correctly transmits a data frame to the reception side communication apparatus, the transmission side communication apparatus sometimes disadvantageously transmits the data frame repeatedly.
In the data communication method employing the ARQ transmission shown in FIGS. 1A and 1B, if the reception side communication apparatus frequently fails to correctly receive a transmitted data frame of a certain number from the transmission side communication apparatus at the beginning of the successive transmission of the data frames, then the reception side communication apparatus continues transmitting the feedback frame including the same repeat request number to the transmission side communication apparatus until correctly receiving the data frame of the requested frame number, thereby preventing the transmission side communication apparatus from determining whether or not the reception side communication apparatus could correctly receive all the data frames to be transmitted. Therefore, after the transmission side communication apparatus transmits all the data frames at least once, the transmission side communication apparatus must repeatedly transmit the data frames once transmitted until it can confirm correct reception thereof of all the data frames to be transmitted.